Barium strontium titanate (BST), which is an oxide ceramic, has a high dielectric constant and excellent ferroelectric and paraelectric properties. As a result, BST is extensively used in the electronics industry for dynamic random access memory (DRAM), ferroelectric, and paraelectric devices and in other applications where a high dielectric constant material is useful.
Applicant Ser. No. 09/300,962, filed Apr. 28, 1999, entitled "Voltage Variable Capacitor (Varactor) for High Frequency Power" describes a BST varactor and its application. The BST varactor was made using 2-ethylhexanoate acid salts.
Thin films of barium strontium titanate have been extensively studied for application in high density dynamic random access memories, monolithic microwave integrated circuit capacitors, tunable microwave filters, and phased array antennae.
Thin films of BST from 2-ethylhexanoate acid salts are also described in U.S. Pat. No. 5,514,822.
The synthesis of thin film ceramic from metal organic acid salts (most usually aliphatic acids such as neodeacanoic or 2-ethylhexanoic) is described in the above-referenced application Ser. No. 09/300,962; application Ser. No. 08/863,117, filed May 27, 1997, entitled "Process for Making Stoichiometric Mixed Metal Oxide Films"; Mantese, Micheli, Hamdi and West in MRS Bulletin, 1989, pp. 48-53; PCT Publication WO 93/12538 (Applicant: Symetrix Corporation); U.S. Pat. Nos. 5,434,102 and 5,439,845, issued to H. Watanabe et al; and C. M. Vest et al, Materials Research Society Symposium, Vol. 60 pp. 35-42 (1986).
The use of anhydride of 2-ethylhexanoic acid to better control the reaction of metal alkoxides to metal salts is disclosed and claimed in U.S. Pat. No. 5,721,009, issued Feb. 24, 1998.
Many of the foregoing references, while providing improved synthetic routes for preparing barium strontium titanate, nevertheless use solvents, such as xylenes and n-butyl acetate, that are toxic or not environmentally friendly.
A review of the need for environmentally benign ceramic precursors and the alumina precursor made from 3,6-dioxaheptanoic acid and an alumina mineral (Group III precursor) is described in "Aqueous Synthesis of Water-Soluble Alumoxanes: Environmentally Benign Precursors to Alumina and Alumina Based Ceramics", Chemical Materials, Vol. 9, No. 11, pp. 2418-2433 (1997).
A paper by A. M. Bahl et al, "Heavy Alkaline Earth Polyether Carboxylates", Inorganic Chemistry, Vol. 36, pp. 5413-5415 (1997) describes the synthesis of barium and strontium oxyacid complexes from the metal hydroxides. The reaction is long and produces water as a by-product, which may not be desired for some applications. The present invention provides for an anhydrous method to produce these compounds.
In a related patent application, "Polyether Acid Anhydride Useful for Improved Non-Toxic Solvent Soluble Group IV and V Metal Acid Salt Complexes", the titanium salt of 3,6-dioxaheptanoic acid is described; see, application Ser. No. 09/307,589, filed on May 7, 1999.
Thus, there is a need for providing improved quality Group II and Group IV or Group V metal oxide films, such as BST films, with less toxic solvents.